1. Field of the Invention
This invention relates to a crankpin phase indexing apparatus and a phase indexing method applied to a crankpin grinding machine for machining, for example, grinding or polishing, a crankpin of a crankshaft.
2. Description of the Related Art
In this type of crankpin phase indexing apparatus, in general, two spindles are rotatably supported by a pair of head stocks, respectively, in axial alignment with each other, and chucks for holding journals on both ends of a crankshaft at locations spaced from the axis of the spindles are mounted to the inner ends of the respective spindles. Spindle driving motors are coupled to the pair of head stocks, respectively. As the spindles are rotated by the respective spindle driving motors, the crankshaft turns around its crankpin that is located in axial alignment with the spindles, whereby the outer peripheral surface of this crankpin can be subjected to machining such as grinding or polishing by means of a rotating grinding stone.
The crankpin phase indexing apparatus includes a phase indexing motor and a phase indexing shaft arranged in association with one of the chucks. As the phase indexing shaft is rotated for indexing by the phase indexing motor, the crankshaft rotates about the journals so that crankpins of the crankshaft, which have different phases, may be selectively set in the machining position and aligned with the axis of the spindles.
In such crankpin phase indexing apparatus, during phase indexing, the phase indexing shaft alone is rotated with the spindles stopped. During the machining operation, on the other hand, the phase indexing shaft is required to be rotated together with the spindles so that the phase indexing shaft is held in a fixed position in relation to the spindles.
To this end, in a conventional phase indexing apparatus, a complicated planetary gear mechanism is interposed between the phase indexing shaft and the spindle so that one of two modes, that is, either the phase indexing by means of the phase indexing shaft or the machining operation, may be selected through operation of the planetary gear mechanism, as disclosed in Japanese Unexamined Patent Publication No. 7-60623, for example.
The conventional phase indexing apparatus, however, has the problem that the apparatus is complicated in structure and is large in overall size, because the planetary gear mechanism includes numerous gears. Also, there is the problem that the manufacturing cost is high. In addition, since the gears in the planetary gear mechanism clatter or chatter, loud noise is produced, deteriorating the crankpin machining environment. Also, the machining accuracy is lowered due to wear of the gears, and maintenance such as lubrication of the gears is required.
When the crankshaft is replaced with one having a different crank arm length, the chucks are adjusted to change the distance between the axis of each spindle and the axis of the corresponding chuck. To permit the distance to be changed, a link coupling, for example, a Schmidt coupling (trade name) 201 shown in FIG. 11, is arranged between the spindle and the chuck.
The Schmidt coupling 201 comprises a driving coupling plate 202 aligned with the axis of the spindle for rotation together therewith, an intermediate coupling plate 203, and a driven coupling plate 204 aligned with the axis of the chuck for rotation together therewith. The intermediate coupling plate 203 is coupled at one side to the driving coupling plate 202 by links 205, and is coupled at the other side to the driven coupling plate 204 by links 206.
As shown in FIG. 12, the center .alpha. of the driving coupling plate 202 and the center .beta. of the driven coupling plate 204 coupled to the driving coupling plate 202 via the intermediate coupling plate 203 are located on a line .gamma., which extends in the direction in which the center distance .epsilon. between the axes of the spindle and the chuck may be increased or decreased.
Usually, in this Schmidt coupling 201, a minimum specified value .delta. is set to define an unstable region with regard to the approaching of the centers .alpha. and .beta., and the distance between the centers .alpha. and .beta. is limited so that it may not become smaller than the value .delta.. If the distance between the centers .alpha. and .beta. is smaller than the minimum specified value .delta., then the links 205 and 206 exceed the dead point, so that the position of the intermediate coupling plate 203 becomes unstable, which causes vibration.
Accordingly, consideration must be given to the space where the Schmidt coupling 201 is installed, and also, the vibration produced is transmitted to the entire Schmidt coupling, which adversely affects the machining operation. With the conventional arrangement shown in FIG. 12, therefore, when increasing or decreasing the center distance .epsilon. between the spindle and the chuck in accordance with a change in the crank arm length of the crankshaft, there is a restriction on the range within which the Schmidt coupling 201 can satisfactorily follow the increase or decrease of the center distance.
Specifically, the distance .epsilon. between the first axis .alpha., which is the axis of the spindle, and the second axis .beta., which is the axis of the chuck, cannot be made smaller than the value corresponding to the aforementioned minimum specified value .delta.. Therefore, the driven coupling plate 204 can be moved toward the driving coupling plate 202 up to a limit position .theta. indicated by the two-dot-chain line in FIG. 12, and the coupling cannot be used with a crankshaft, the crank arm length of which is smaller than the above value .delta..
Generally, a crankpin, as a workpiece, is low in rigidity; therefore, in this type of crankshaft grinding machine, the crankshaft must be rotated so that it does not undergo twisting or the like during the machining operation. To this end, a synchronous driving unit has conventionally been proposed in which the two spindles are operatively coupled to each other by a synchronizing shaft so that synchronous rotation of the two spindles is achieved by the synchronizing shaft.
In this conventional synchronous driving unit, however, since the two spindles must be rotated exactly in synchronism with each other via the synchronizing shaft, it is necessary that labor-consuming adjustments be performed in order to, for example, eliminate the backlash of the rotary coupling parts, and also, a large-diameter, high-rigidity synchronizing shaft must be used to prevent the twisting thereof. In addition, after long use of the unit, its rotary driving system may become loose due to wear or the like, possibly making the spindles fail to rotate in perfect synchronism.
To cope with these problems, another type of synchronous driving unit has conventionally been proposed in which separate motors are operatively coupled to the respective spindles and are subjected to synchronous rotation control. With this conventional unit, however, the two motors may possibly fail to rotate synchronously due to abnormality or the like of the control system associated with one of the motors. If the synchronous rotation fails, then the crankshaft will be twisted and the machining accuracy will deteriorate.
To solve the problem, still another type of synchronous driving unit having an arrangement as shown in Japanese Utility Model Examined Publication No. 7-23945, for example, has conventionally been proposed. In this conventional arrangement, the two spindles are synchronously rotated by a pair of motors, respectively, and are rotatably interlocked by a security shaft having an axis displaced from the axis of the spindles. If the synchronous control fails due to a motor fault or the like, rotation of one spindle is transmitted to the other via the security shaft, whereby the crankshaft is prevented from being excessively twisted.
In this conventional synchronous driving unit, when the synchronism of the two spindles is lost due to an abnormality or the like of the motor control system, the rotational force of one spindle is used to forcibly rotate the other spindle via the security shaft, thereby mechanically ensuring the synchronism of the two spindles. Accordingly, in the event the synchronism is lost, the security shaft is required to transmit torque and thus must be thick and firm. This, however, entails a significant increase in the weight of the security shaft, and also, during normal operation, in which the security shaft need not function, the heavy security shaft consumes motor power, thus causing energy loss.
Further, in the case where the synchronism of the two spindles is lost due to an abnormality or the like of the control system of one motor, the opposite ends of the crankshaft are acted upon by different magnitudes of driving torque, and as the crankshaft in this state is forcibly rotated by the security shaft, it becomes twisted. If the machining operation is continued with the crankshaft twisted, the machining accuracy may greatly deteriorate.